The present invention relates to a so-called direct-injection engine (it's also called as “in-cylinder direct injection type internal combustion engine”) in which a fuel (for example, gasoline) is directly injected into a cylinder and also relates to a method of controlling the engine, a piston used in the engine, and a fuel injection valve used in the engine.
A direct-injection engine, disclosed in Japanese Unexamined Patent Publication No. 2000-130171, is based upon a direct injection/spark ignition type internal combustion engine, and which is provided with two intake valves and two exhaust valves at a cylinder head forming a pent roof type combustion chamber. Furthermore, the engine is provided with a spark plug at a substantial center of the cylinder head and a fuel injection valve for directly injecting a fuel into the cylinder. The fuel injection valve is arranged on the side of the intake valves in a state of being directed toward the center of the cylinder. In this arrangement, stratified combustion is realized by injecting the fuel at a compression stroke in a state where a tumble flow component of intake air is formed in the cylinder. In this prior art, a bowl, which has a cylindrical surface or spherical surface extending along a piston diameter line nearly orthogonally crossing with a fuel spray axis, is recessed on the center of a piston crown surface. Furthermore, a recessed portion recessed more deeply than the bowl is formed in a range from the intake valve side of the piston crown surface to the nearly center of the piston so that a fuel spray injected in the vicinity of a top dead center at a compression stroke is guided to the side of the spark plug.
However, as a normal conical spray with nearly uniform distribution in a circumferential direction is used in the above-mentioned configuration, a penetration of the spray is relatively small. Therefore, at the vicinity of the top dead center in the compression stroke such as approximately at 30 degrees before the top dead center, a spray angle is narrowed, and the whole quantity of the spray is injected toward the piston. Therefore, the recessed portion on the piston crown is required to be made deep so that the depth is equivalent to approximately a quarter of the height of the cylinder between 30 degrees and 10 degrees before the top dead center in a crank angle.
According to the above configuration, as the bowl is relatively deep, and the additional recessed portion deeper than the bowl is provided, the surface area of the piston increases. As a result, particularly in a high-load condition of the engine, output and fuel economy of the engine are deteriorated due to the increase of cooling loss is caused.